1. Field of Invention
The present invention relates to irradiance, and more particularly to methods and apparatus for producing electromagnetic radiation.
2. Description of Related Art
Arc lamps have been used to produce electromagnetic radiation for a wide variety of purposes. Generally, arc lamps include continuous or DC arc lamps for producing continuous irradiance, as well as flashlamps for producing irradiance flashes.
Continuous or DC arc lamps have been used for applications ranging from sunlight simulation to rapid thermal processing of semiconductor wafers. A typical conventional DC arc lamp includes two electrodes, namely, a cathode and an anode, mounted within a quartz envelope filled with an inert gas such as xenon or argon. An electrical power supply is used to sustain a continuous plasma arc between the electrodes. Within the plasma arc, the plasma is heated by the high electrical current to a high temperature via particle collision, and emits electromagnetic radiation, at an intensity corresponding to the electrical current flowing between the electrodes.
Flashlamps are similar in some ways to continuous arc lamps, but differ in other respects. Rather than using a constant electrical current to produce a continuous radiant output, a capacitor bank or other pulsed power supply is abruptly discharged through the electrodes, to generate a high-energy electrical discharge pulse in the form of a plasma arc between the electrodes. As with continuous arc lamps, the plasma is heated by the large electrical current of the discharge pulse, and emits light energy in the form of an abrupt flash whose duration corresponds to that of the electrical discharge pulse. For example, some flashes may be on the order of one millisecond in duration, although other durations may also be achieved. Unlike continuous arc lamps, which typically operate under quasi-static pressure and temperature conditions, flashlamps are typically characterized by large, abrupt changes in pressure and temperature during the flash.
Historically, one of the major applications of high power flashlamps has been laser pumping. As a more recent example, a high power flashlamp has been used to anneal a semiconductor wafer, by irradiating a surface of the wafer at a power on the order of five megawatts, for a pulse duration on the order of one millisecond.
Cooling of conventional flashlamps typically consists of cooling only the outside surface of the envelope, rather than the inside surface. Although simple convection cooling using ambient air is sufficient for low-power applications, high-power applications often require the outside of the envelope to be cooled by forced air or other gas, or by water or another liquid for even higher-power applications.
Such conventional high-power flashlamps tend to suffer from a number of difficulties and disadvantages. One factor that tends to limit the lifetime of such lamps is the mechanical strength of the quartz envelopes, which are typically on the order of 1 mm thick, and rarely exceed 2.5 mm in thickness. In this regard, although increasing the thickness of the quartz envelope increases its mechanical strength, the additional quartz material provides added insulation between the cooled outer surface of the envelope and the inner surface of the envelope, which is heated by the plasma arc. Therefore, with thicker tubes, it is more difficult for the outer coolant to remove heat from the inner surface of the envelope. As a result, the inner surface of a thicker envelope is heated to higher temperatures, resulting in greater thermal gradients in the envelope which tend to cause thermal stress cracks, ultimately leading to envelope failure. Thus, the thickness of an envelope, and hence its mechanical strength, are limited in conventional flashlamps. This in turn limits the ability of the envelope to withstand the mechanical stresses resulting from the significant rapid changes in gas pressure within the envelope resulting from the rapid increases of arc temperature and diameter during the flash.
A further difficulty with conventional lamps involves ablation of the quartz envelope, primarily from evaporation of quartz material from the heated inner surface of the envelope. Such ablation tends to contaminate the arc gas with oxygen. As most commercially-available arc lamps are sealed systems rather than recirculating, the accumulation of such contaminants in the arc gas tends to cause the radiant output of the lamp to drop over time. Such changes in the radiant output of the flashlamp may be undesirable for many applications, such as semiconductor annealing, in which reproducibility is strongly desired. The accumulation of these contaminants also tends to make the lamp more difficult to start.
Yet another disadvantage of conventional flashlamps results from sputtering of material from the electrodes, which are typically made of tungsten or tungsten alloys. In this regard, the abrupt emission of electrons and the resulting arc can sputter or blast off significant amounts of material from the cathode. To a lesser extent, the abrupt electron bombardment and the heat of the arc can cause partial melting of the anode tip, also resulting in the release of anode material. As a result, sputtering deposits tend to accumulate on the inside surface of the envelope, thereby reducing the radiant output of the lamp, as well as causing its radiation pattern to become increasingly non-uniform over time. In addition, such deposits on the inside surface of the envelope tend to be heated by the flash, thereby increasing local thermal stress in the envelope, which may eventually lead to cracking and failure of the envelope. Such loss of material also reduces electrode lifetimes.
A further disadvantage of conventional flashlamps is the relatively poor reproducibility of the radiant emissions of the arc itself. Some conventional lamps maintain a low-current continuous DC discharge between the electrodes, referred to as an idle current or simmer current, in between flashes. The purpose of the simmer current in conventional lamps is primarily to heat the cathode sufficiently to begin emitting electrons, which reduces sputtering and thereby increases lamp lifetime, although the simmer current may also provide at least some pre-ionization of the gas. The simmer current is typically less than one amp, and generally cannot be significantly increased in conventional flashlamps without causing overheating of the electrodes and sputtering. As a result, the present inventors have observed that the large change in the arc current that occurs in the transition from the simmer current to the peak flash current tends to occur in a relatively inconsistent manner in conventional flashlamps, resulting in poor reproducibility characteristics of the flash.
Accordingly, there is a need for an improved flashlamp and method.